Read this comment (or thread) as to why not BLAKE3: https://news.ycombinator.com/item?id=29746938

To sum up: BLAKE3 has no advantages over BLAKE2 when it comes to the Linux kernel[1], plus BLAKE2 is already in the kernel (with tests and everything), while BLAKE3 is not (at all). That, and BLAKE3 is still fairly new.

[1] It is faster than BLAKE2 due to parallelism, which is not suitable for the Linux kernel; you want to restrict computation to a single thread instead of making all the cores busy. FWIW, the C reference implementation is not multi-threaded either.